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Nice Goldies =) You mentioned in one post skin irritations and such which got me thinking .... Then I noticed you have a Pleco. Plecos and Goldies do NOT mix. As the Pleco gets bigger it starts to attack Goldies. Something they find appealing about the slime coat of Goldies. Usually won't see it happen 'cause Plecos are more active at night. Sleeping and fancy Goldies are easy targets for them.

Best bet is to get rid of the Pleco and replace it with far better algae eaters (as I see you have algae problems anyway which means your Pleco is eating something other than algae)... Olive Nerite snails are much better algae eaters and do not multiply in freshwater and dine almost exclusively on algae and do not attack fish fins. Plecos eat algae if there's nothing else to eat only. I'd get rid of the assassin snails, too... If your Goldies sleep on the bottom of the tank (as many Goldies do), the assassin snails attack their tails and munch on them -- eventually the Goldies will get infections from that behavior. If you're not overfeeding the tank, there's no reason for a snail outbreak and no need for killer snails. The brown algae you have all over the place also suggests water quality issues -- probably insanely high phosphates from overfeeding. Granted, overfeeding and Goldies tend to go hand-in-hand.

Plastic plants... The grassy plastic plants you have have sharp ends to them with Goldies tend to snag their fragile fins on and Orandas (which tend to have problems seeing once their ridiculous fleshy head growths take over) tend to injure their fleshy growths on. I'd replace those... Silk plants are actually the best, most natural looking, and Goldie favored artificial plants to use. They look better and move in the current like live plants would. The placement of the plants doesn't offer much in the way of security for the fish either... Group them along the back in bunches so they can actually offer hiding places for the fish, leaving the front and center of the tank open for fishy activities. The pot you have in there would be more helpful to them if the open end were turned away from view to add a feeling of security. Also note that pots like that create dead spots in water circulation -- especially problematic when you have fish that are almost constantly being fed something.

But, again, nice Goldies and great photos toward the end of your posts =)

If you read a little more carefully, you'll note that this thread covers (or did cover) multiple tanks. The goldfish are not in a tank with the pleco or snails, and have not been for a year and a half.

For the sick fish with unknown issues.... Pick up some PraziPro medicated food and treat it. Likely has either internal parasites or internal infection(s) or both. If you're going to keep a highly social fish like that in quarantine, it should be for treatment only. Remove all other food sources while treating and add proper filtration (if you haven't already). Treat for at least 14 days. Keep his water temp. higher, too... Fancy Goldie prefer 72-74˙F for optimal health. Feed him only essential veggies 1-2 times a week at most (essential being greens like peas or cucumbers, etc.). Do NOT feed broccoli or other gas inducing foods while treating the fish! Change the water no more than once a week and no more than 25% -- water stability is important for sick fish. Add some plants of something so the poor thing has a sense of security, too... Completely open tanks, especially bare bottom ones, are highly stressful to fish. When you do the water changes, you can remove the plants and put them through a quick chlorine dip to disinfect them and then put them back in. Add a lot of O2 to high water as well.... I'm talkin', bubble wall along the whole back of the tank and turned up as high as the fish can tolerate the current -- meaning that adjustments downward in airflow should be made to just the point where the fish doesn't have trouble fighting the current. If you have access to it, buy some Anacharis and provide it for the fish at all times. The fish should munch on it (once it figures out its food) and will be provided with a high dose of vitamin C and the natural antibiotics the plants naturally contain. Keep the excess plants in a separate container with proper plant lighting and nutrients to grow so you have a source for fresh plants to feed to the fish (away from your other fish!). Another very beneficial thing to give the fish is a garlic supplement. There are a few on the market, fairly easy to find... While in quarantine, you can give it 2x the normal dose on the package.

One telling thing to watch in sick fish -- as crazy as it sounds -- is their poop. The color and consistency can be a very good indicator of internal problems.

Anyway.... Hope some of the above info. is helpful if your fish is still sick.